This invention relates to an improved fuel element for fast spectrum nuclear reactors. More particularly, the invention relates to a fuel element having means for protecting the cladding against damage thereto from ceramic chips which may be expected to chip off of the fuel pellets internal of the cladding.
Conventional fast breeder reactor fuel elements consist of ceramic fuel pellets stacked in a column and inserted into a metal tube or cladding such as stainless steel which is welded closed with metallic end plugs. Due to the high temperature at which fast reactors operate, it is essential to have efficient heat transfer between the nuclear fuel and the cladding. For this reason, it has been customary to place in the intervening space a heat transfer agent such as sodium or some other metal that is liquid at reactor operating temperatures. Such a fuel element is referred to as a "sodium bonded" fuel element.
During operation of the reactor, substantial thermal stresses are caused by periodic heating and cooling of the reactor. These thermal stresses may produce cracks in the ceramic fuel pellets. When this happens, small pellet chips may become wedged between the larger chips and the clad. As the fuel pellets subsequently swell due to fission product buildup, these small chips are driven into the clad producing localized high stresses and localized points of weakness. It is believed that these localized effects caused by sandwiching the small chips between the swelled fuel pellet and the clad is one cause of clad failure. Such failure may release radioactive fission products to the coolant circulating around the fuel element and through the reactor system: a result which obviously should be avoided.
In order to address the cladding failure problem originating from chips of fuel pellets being pressed into the cladding by the swelling pellets, the conventional solution of placing a very thin-walled tube (approximately 0.003 inches) with a very small tube to pellet diametral clearance (less than 0.003 inches) within the annular region between the pellets and cladding was devised. However, in order to provide for ingress of the bonding sodium, it usually is necessary to puncture the thin-walled tube prior to pellet loading to form a series of small perforations.
The above described solution has not been without its difficulties. It has been found that the fabrication and assembly of such a fuel element is both difficult and expensive. The tight specifications required for the thin-walled tubing are not easily or inexpensively met. Further, the task of loading the pellets into the thin-walled tubing with such a small diametric clearance between the pellets and the tube itself, has proven to be extremely difficult. Finally, it has been discovered that the above described solution is less than adequate since the small perforations serve as sources for crack initiation and propagation with consequent loss of restraint capability.
U.S. Pat. No. 3,067,116 discloses an approach similar to that described above in which the column of pellets are wrapped lengthwise with a thin sheet of pre-slit metal. The claimed purpose for such thin sheet pre-slit material is to provide a spacer which functions to maintain the space between the fuel pellet and the cladding itself. In the form disclosed, however, the spacer has been found to be less than effective as a pellet chip restrainer since the wrapped configuration did not provide a circumferentially continuous cylindrical sleeve but provided a sleeve with a sliced area or an overlapping area where the opposite edges of the thin-sheet material meet. Additionally, the pre-formed slits in the thin-sheet material provide abundant sites for the beginning and propagation of cracks: which cracks serve to prevent the sheet of material from acting as an effective chip restraint device.